In manufacturing a liquid crystal display panel, two glass substrates with a transparent electrode, a thin film transistor array, and the like need to be assembled together with a sealing material by leaving a very narrow space in the order of several μm between each other, and by filling the space with liquid crystal and sealing it.
Here, conventionally, for example, a method described below is used for filling and sealing the liquid crystal.
Firstly, under ordinary pressure, a sealing material having an opening for injecting the liquid crystal is formed around the periphery of a cell in one of two substrates, and then the two substrates are assembled, pressed, and hardened. Next, the assembled substrates are cut into a predetermined cell size or the like, so that the injection opening is on the glass edge. After that, the cells of a predetermined cell size are sealed after injecting the liquid crystal through the injection opening by a conventional liquid crystal injection method.
In the method for injecting the liquid crystal the step of assembling the two substrates and the step of injecting the liquid crystal needs to be separately performed. However, in recent years, the step of assembling the two substrates together and the step of injecting the liquid crystal have been performed simultaneously. This is enabled by assembling the two substrates in a vacuum after dropping the liquid crystal onto the substrate on which the sealing material has been applied.
Specifically, the injection of liquid crystal in the vacuum device used in this example is carried out in a vacuum so that the two substrates are assembled together by applying the sealing material to one of the substrates that is to be assembled, and by dropping the liquid crystal onto one of the substrates.
Incidentally, when the liquid crystal is dropped and the substrates are assembled in a vacuum as above, the following problems are caused.
First, a vacuum suction method cannot be used for holding an upper substrate, because the injection step is performed in the vacuum device. Also, in cases where an electrostatic chuck is used to hold the substrate, characteristics of TFTs (Thin Film Transistors) formed on the substrate are changed due to a high voltage applied when holding the substrates.
To solve these problems, for example, Japanese Laid-Open Patent Application Tokukai 2001-133745 (published on May 18, 2001) proposes an adhesive material such as an adhesive sheet for holding the substrates in a vacuum.
This method ensures the substrates to be easily assembled together in vacuum with high accuracy even for large and thin substrates.
However, in the conventional method disclosed in this publication, a type of adhesive sheet used for holding the substrate in a vacuum is not specifically described.
Generally, the adhesive sheet may be a tape or the like to which an adhesive agent has been applied. However, in cases where the substrate is held by such a tape, there is a problem that the adhesive agent remains on the substrate. Further, in cases where the adhesion is too strong, the adhesive sheet cannot be detached from the substrate. This may cause damage on the substrate when the adhesive sheet is detached from the substrate.
Further, the upper and lower liquid crystal glass substrates are assembled together with a space as narrow as 2 μm to 9 μm between the substrates. This affects the performance and quality of the liquid crystal display panel. For example, the brightness of the liquid crystal display may be affected by the distance of the space. Therefore, it is important to assemble the liquid crystal glass substrates so that the substrates have an even thickness, and that pressure is exerted evenly over the entire substrates. However, because the adhesive tape or the like does not have even thickness, it is impossible to substantially evenly exert pressure over the entire substrates.
The present invention is made in light of the conventional problems, and an object of the present invention is to provide a method for holding a substrate in a vacuum, a method for manufacturing a liquid crystal display device, and a substrate holding device, whereby the adhesive agent is prevented from remaining on the substrate, and the adhesive sheet is detached from the substrate after the assembly.
Another object of the present invention is to provide a method for holding a substrate in a vacuum, a method for manufacturing a liquid crystal display device, and a substrate holding device, whereby pressure is evenly exerted over the entire substrate, and the substrate is assembled with the other substrate with an even space between each other.